The committee’s chairman insists the hearing isn’t explicitly about President Donald Trump, who has repeatedly insulted and threatened the leader of another nuclear-armed power.

Instead, says retiring Sen. Bob Corker, R-Tennessee, it’s about the balance of power between the branches of government more broadly.

“A number of members both on and off our committee have raised questions about the authorities of the legislative and executive branches with respect to war making, the use of nuclear weapons, and conducting foreign policy overall,” Corker said in a statement before the hearing.

But critics and opponents of Trump have questioned his suitability to command the United States’ substantial nuclear arsenal. And Corker has not shied away from criticizing the president’s foreign policy, saying last month that Trump was setting the U.S. “on the path to World War III.”

So if Congress hasn’t held a public discussion about nuclear authority since Leonid Brezhnev led the Soviet Union, was it Trump that brought the issue to mind?

“People are concerned about this particular president, but what this has done is uncover this larger question of ‘Why do we have this crazy system?” said Cirincione, president of the arms control think tank Ploughshares Fund. “Why do we have it built this way?”

Does it matter why a president is launching them?

Essentially, there are two scenarios in which a president would order the use of nuclear weapons: an adversary has already launched one or more ICBMs, or a preemptive “first-use” strike.

In the former scenario, there’s little time to consult wide swaths of the president’s national security team, let alone Congress. Decisions have to be made within minutes. The president would likely attempt to convene a conference of a few top advisors. The chain of command has to be short to activate any kind of meaningful response, said Brig. Gen. Gregory Bowen, the deputy director of global operations at U.S. Strategic Command.

“It is designed the way it is for a reason, and that is to be able to rapidly respond in an in extremis situation,” Bowen told the Defense One Summit audience. “There’s missiles inbound; you’ve got to do something very rapidly. But having said that, it is a very tightly scripted process.”

The latter scenario — a president orders a nuclear strike on a country that has not yet launched its own weapons — is what lawmakers are currently more concerned about.

“The president has sole authority and unilateral power to order the use of nuclear weapons,” said Bruce Blair, a nuclear command-and-control expert at Princeton University and nonproliferation advocate. “But there’s many in Congress who think that the president should not be allowed to order the launch of nuclear weapons in any circumstance short of a confirmed attack against the United States.”

Obama administration officials considered saying the U.S. would never be the first to launch nuclear bombs in conflict, but ultimately decided to keep the possibility on the table. The Trump administration’s first Nuclear Posture Review is still underway, but is not expected to change that policy.

It’s important not to perpetuate a “mythology that the president can wake up and press a button and off goes a nuclear weapon,” said Troy Thomas, a Boston Consulting Group associate director who has served on the National Security Council. In any first-use case, Thomas said, there’d be a conversation with advisors of the president’s choosing, and the officials implementing the strike would have to weigh whether the order was legal under the law of war.

But as tensions with North Korea cycle up and down, foreign policy experts worry about a miscalculation on either side.

“Most people are concerned a president might order the use of nuclear weapons in circumstances that aren’t compelling, that his advisors would find such a decision to be misguided and a terribly bad call,” Blair said. “But he would still be within his rights to order the use of nuclear weapons against a country that he considered to be a threat to the United States.”

How much authority can Congress actually wield?

Even as Corker hosts tomorrow’s hearing, there’s a question of how much say lawmakers can legally demand. The concentration of authority in the White House isn’t just for operational or deterrent effects; it’s also a constitutional matter, Thomas said.

“The president of the United States has always had the authority to use the military instrument of power,” he said. “And the nuclear weapon is just one weapon in there — it’s the most destructive and potentially devastating weapon in the arsenal, but there’s also some significant conventional capabilities as well.”

Blair pointed out that Congress could pass a law banning first-use, and some lawmakers — Sen. Ed Markey, D-Mass., and Rep. Ted Lieu, D-Calif., — have suggested just that.

“Congress has a lot more standing to pass such a law than it does to interfere in the chain of command in the executive branch by saying the secretary of defense has to be in the chain of command” for launching a strike, he said.

But in practice, distinguishing between a defensive and offensive use of force would be difficult. And for the Pentagon, it’s ultimately a military decision:

“At the end of the day, the president of the United States is the commander-in-chief and if he gives me a lawful order I will execute it,” Bowen said.

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